Method of developing an electrophotographic image with a bias field

ABSTRACT

A method of developing electrostatic images comprising subjecting a surface containing an electrical image to a liquid developer having two dissimilar components, one of which is a first developer component having a polarity such that it develops the image by attracting the first developer component to the image. A bias field is maintained through the liquid such that as the image is developed by the first developer component the second developer component is moved away from the surface being developed. When the first developer has been differentially developed on the surface the polarity of the bias field is reversed to cause the second developer component to be deposited on the first developer component.

United States Patent 1191 Metcalfe et al.

[ 1 Sept. 17, 1974 W. Wilson, Oaklands Park, all of Australia [73] Assignee: The Commonwealth of Australia,

care of the Secretary, Department of Supply, Parker, Canberra, Australia [22] Filed: Oct-2, 1972 [21] Appl. No.: 294,000

[30] Foreign Application Priority Data Sept. 30, 1971 Australia 6478/71 52 US. 01 96/1 LY, 96/1 R, 117/37 LE, 1 252/62.1

51 1111. 01.... G03g 9/04, G03g 13/10, 003 15/10 58 1 16111 61 Search 117/37 LE, 17.5; 96/1 R, 96/1 LY; 252/62.l L

[56] References Cited UNITED STATES PATENTS 3,010,842 11/1961 Ricker ..252/62.1L

3,081,263 3/1963 Metcalfe et al. 252/62.l L 3,094,429 6/1963 Howell 252/62.l L 3,300,410 1/1967 Oliphant 252/62 1 L 3,337,340 8/1967 Matkan 252/62.l L 3,679,453 7/1972 Katagiri et a1. 117/37 LE Primary Examiner-Ronald H. Smith Assistant Examiner-J. P. Brammer Attorney, Agent, or Firm-Waters, Roditi, Schwartz & Nissen [57] ABSTRACT A method of developing electrostatic images comprising subjecting a surface containing an electrical image to a liquid developer having two dissimilar components, one of which is a first developer component having a polarity such that it develops the image by attracting the first developer component to the image. A bias field is maintained through the liquid such that as the image is developed by the first developer component the second developer component is moved away from the surface being developed. When the first developer has been differentially developed on the surface the polarity of the bias field is reversed to cause the second developer component to be deposited on the first developer component.

6 Claims, 2 Drawing Figures j 6 O O 0 o 7 I 23% A (a me 5915 Mm/8 METHOD OF DEVELOPING AN ELECTROPHOTOGRAPHIC IMAGE WITH A BIAS FIELD This invention relates to a method of and means for developing electrophotographic images and refers to both normal charged images and also uncharged images.

BACKGROUND OF INVENTION The development of electrophotographic images usually comprises the charging of a surface which consists of a photoconductive medium such as zinc oxide set in an insulating resin, or selenium or any of the sulphur compounds used as a thin layer, and then exposing the layer to light after charging to bleed away the charges in those areas where the image is light struck, after which the surface, now containing a latent electrostatic image in the dark areas is subjected to a developer which is selective to the charged surface and deposits thereon. Instead of using light it is possible to use other electromagnetic waves such as X-rays.

Developing of the image can be by means of dry developer powders, either used as a powder cloud or carried over the surface by means of granules which give to the developer powders a triboelectric charge, or the developer can be a liquid developer in which the marking particles are suspended to be freely mobile in a carrier liquid having a resistivity sufficiently high to prevent destruction of the latent electrostatic image during development, such as for instance as cm. and a dielectric constant less than 3, the liquid development process being generally improved by use of a control medium which regulates the movement of the toner particles to ensure that correct deposition takes place.

It has also been proposed to use development, known as chargeless development, in which the photoconductive surface is simply subjected to a pattern or image by appropriately subjecting the surface to electromagnetic waves, and then, by using an extremely sensitive developer, developing the latent image so produced. An image so produced is of the lower density because of the reduced charge available to control the movement of the developer particles, but nevertheless, with the liquid developers now available, this latent image can be rendered visible.

The present invention applies to either type of process, namely that known as chargeless development or that using a precharging of the surface but by use of the present process much more effective development of chargeless images is possible due to the denser images which can be obtained and also due to the much better control which the present invention can apply generally. In the case of charged surfaces this is still however true and the invention will be seen to have substantial merit with this type of process also.

SUMMARY OF INVENTION The present invention is directed to development of a surface which has on it an image produced either by precharging and modifying by electromagnetic waves or by simply modifying by electromagnetic waves, and then developing by application of a liquid developer in which marking particles are suspended in the carrier liquid of such a developer, the invention utilizing a primary developer component and a secondary developer component with two stage biasing during development to deposit the two components sequentially the first biasing being carried out with a polarity such that the secondary component which comprises toner particles are moved away from the surface to be developed while the primary developer component develops the image, after which the polarity is reversed and the toner particles are driven down on to the primary developer component.

The photoconductor surface is such that where activated by electromagnetic waves, the photoconductive surface is in a conductive state but where not so activated in an insulating state,

The developer is selected to contain two components, one being more insulating than the other, the more insulating component being inherently attracted to the insulated field bearing area of the photoconductor because of a selected opposite charge potential, the other relatively more conductive component being such that it is not retained on either a remote electrode unless it is insulated or on the conductive part of the photoconductor surface because of charge exchange (it being capable of having either a positive or negative characteristic according to this) so that development is effected by first lightly developing the image by the first insulating component while the bias removes the other more conductive component from the photoconductive area, and then intensifying the development by re versing the electrode polarity which will allow the sec ond developer to be built up on the first developed area only because this is the only area where charge exchange does not take place within the time available.

We have found that the invention produces highly satisfactory images and allows required contrast and densities to be obtained not only by a selection of ap propriate developers but by the time element applicable to the movement of the developer during the initial denuding of the face of the photoconductive medium of developer during the initial application of a field and the subsequent reversing of the field to drive down the developer in the required areas.

The invention also has various other advantages which are considered to be important, these being for instance the improvements to fixing which can result by the deposition of a tacky first developer component and also the greater flexibility of development.

Thus during the first cycle the insulating medium can be chosen to develop the latent image on the surface, it having been found that in the case of a photoconduc tive surface, the insulating medium, be it a resin developer component or a liquid component, will be forced down on to the surface if correctly selected and will deposit irrespective of whether the latent image has been formed by precharging and light bleeding or the like or whether the surface has merely been dielectrically modified by simply exposing the surface to electromagnetic radiation of patterned form, or is otherwise dielectrically charged.

Because an insulator has charge carriers not readily available, we have demonstrated that an insulator medium will retain for a longer time whatever charge it has, and therefore because of the lack of readily available charge carriers, when such a surface is deposited on to a surface which is subsequently changed in polarity, it is difficult to drive off these particles because of the difficulty of exchanging charges with the surface on which the particles are deposited.

It follows from this that as insulator particles tend to remain on a surface on which they are deposited by means of an electrical field, that provided the time element of the biasing is correct, it is possible after deposition to change the direction of biasing, that is to change the field, but this will not drive off the deposited insulator medium, and this medium then remains in position in imagewise pattern on the surface where it was first deposited.

When however the change in biasing occurs the particles of toner medium which are relatively more conductive, and thus have charge carriers readily available, are now repelled from the further electrode and are attracted to the surface containing the latent image, development of the image will take place under control of the insulating medium which was deposited in the earlier stage of development.

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a schematic view showing how when a photoconductor membrane 1 is placed into a developer 2 the primary developer which is the insulator medium 3 is drawn to the image area 4 but when the base electrode 5 and distant electrode 6 are energised negatively and positively respectively, the toner particles 7 which form the secondary developer and which have an inherent negative charge are moved toward the remote electrode 6 to denude the area near the negative base electrode 5. The distant electrode bias insulator coating 8 on it to discourouge charge exchange.

FIG. 2 shows how, when the polarity of the electrodes is reversed, the toner particles 7 are drawn down toward the base electrode 5 but can be retained only where they deposit on the insulator image 3, because if they contact the surface of the conductive part of the photoconductive surface they exchange charges under influence of the base electrode 5 and move away as shown in that figure. In this way the invention offers very much more effective control than has been possible heretofore in that a simple developer can be used which consists of insulating particles or an insulating liquid together with relatively conducting developer particles suspended in an insulating liquid, and by so selecting the liquid that the required insulating value is present, this liquid or the insulator medium is first deposited in an imagewise manner and having been deposited is somewhat difficult to remove from the area where it has been deposited, so that when the developer particles can be discharged from the electrode by reversal of the field, are directed towards the image area and can be forced down at the required localities before such time as the insulating medium can be released from its deposited position by such insulating medium taking the charge of the area with which it is in contact.

Thus the slower acting insulating medium is first deposited, and once deposited, development can take place without removing the insulating medium, as the toner particles are more readily moved in the field because of greater conductivity and would on contact with a conducting surface on which they are deposted immediately change their characteristic by charge transfer and would again move away from the deposited site.

In this connection it has been shown that the rela tively conductive particles will alternate between opposite poles at a rapid rate whereas particles of insulating material will tend to take a considerable time to discharge and move to an opposite pole.

This is true of liquid also as has been observed by watching streaming effects in liquid under these conditions.

Where the insulating medium is a resin supported in the carrier liquid, it can be in a partly disolved or a disolved state to act as an adhesive to lock toner particles to the image area when deposited thereon, the fixing becoming effective when the carrier liquid evaporates.

EXAMPLE I A typical photoconductive coating formulation suitable for this application is:-

Pigment: Zinc oxide, Special Z, supplied by Durham Chemicals Co. Aust. Ltd.

Binder: Lustrasol AUISO/SO, Trade Mark Alkyd modified acrylic resin, supplied by Reichold Chemicals of Aust. Ltd.

Additives:

Dyes, Rose Bengal, Erythrosin B, Sodium Fluorescein, Brilliant Green Driers: Cobalt napthenate Solvents: Toluol Diluent: Chlorothene Composition: Zinc oxide 3000 grams Resin binder 1 I25 do. Rose Bengal 005 gram Erythrosin B 0.10 do. Sod fluoroescein 0.30 do. Brilliant green 050 do. Cobalt naphthenate 0.05 percent cobalt metal as naphthenate, calculated on solid I'CSII'I content. Substrate: Metal, paper or film base. Appearance: The finished dry coating is off-white. It is to be remembered that a photoconductive coating as described is insulating where charged and the charge is retained but becomes relatively conductive where light struck. It is important to remember this in relation to the conductive developer and its charge exchange characteristic during the secondreversed bias. A typical first developer component formed as follows:

Developer Component No. 1 (More insulating) A relatively insulating developer concentrate of relatively coarse particle size suitable for dispersion in paraffinic liquids (carrier liquids) such as cyclohexane or Isopar G or Isopar E may be comprised as follows:

The copolymeric resins are relatively positive and tend automatically to deposit on a negative latent image so soon as development takes place.

A second component developer, that is a conductive developer with negative characteristic is formed as follows.

Vinylite polymer VYNW 5 parts by wt (fixing medium) Carbon Black 1 part by wt (colouring medium) Calcium octoate 0.001 parts by wt (conductive medium) Both are suspended in Isopar G or E in proportion of 2 part of conductive to 1 part by weight of more insulating developer.

First biasing positive on remote electrode volts for 30 seconds to assist more insulating medium down on to image area.

Second biasing negative on remote electrode 180 volts for 2 seconds to force toner particles down on to photoconductor but to be held only where there is a deposited layer of the insulating component. Charge exchange takes place directly at these parts of the photoconductor layer where it is in a conductive state.

EXAMPLE 2 In example 1 replace the carbon black by Irgafin S1 trade mark phthalocyanine blue pigment, coated by a copolymeric resin as manufactured by Geigy, Australasia Pty. Ltd.

EXAMPLE 3 In example 1 replace the Pliolite VT by a styrene/acrylate copolymer such as that marketed under the trade mark Pliolite ACL trade mark of Australian Synthetic Rubber, of Altona, Victoria, Australia.

IDENTIFICATION OF TRADE MARKS AND TRADE NAMES MOWITAL B6011, polyvinyl butyral resin, made by Hoechst, Germany; containing polyvinyl acetal 76-78 percent, polyvinyl acetate 1 percent and polyvinyl alcohol 18-21 percent.

ISOJORDOSOL 4501/60 short oil alkyd resin made by Jordan Chemicals BUTON 200, styrene-butadiene copolymer made by Esso SOLPRENE 1205, sytrene-butadiene copolymer Phillips Petroleum Corp, U.S.A., a block copolymer of butadiene and styrene in the ratio 75/25 containing 97.5 percent of rubber hydrocarbon, A.S.T.M. No. 1205 with majority of styrene molecules added as polystyrene at the end of a long chain of butadiene units.

Superbeckosol 1352/60, a semi-drying safflower oil isophthalic-modified long oil alkyd resin with 59-61 percent non-volatile matter, acid value 3-6, oil length 60 percent, viscosity Gardner Holdt Y-Z.

VINYLITE VYNW, a vinyl chloride-acetate resin, approximate composition vinyl chloride 97 percent, vinyl acetate 3 percent, and specific gravity 1.39.

PENTACITE P423 is a modified pentarethyritol ester resin with acid number 20-30.

PLIOLITE VT RESIN is a styrene/butadiene type copolymer rubber made by the Goodyear Corp., U.S.A. and prepared by the G.R.S. method in which the butadiene polymerises in the main by a 1,4-addition. Pliolite VT is a vinyl toluene/butadiene random copolymer rubber, soluble in mineral spirits.

PLIOLITE V.T.A.C. is a vinyl toluene/acrylate co- 5 polymer, KB value 36.

ESSO 100 Solvent is a hydrocarbon solvent supplied by Esso Chemicals Australia Limited, having an aromatic content of 98 percent, flash point of 108F., and

distillation range 159-182C.

MICROLITH Pigments comprise a pigment and a resinous carrier. Microlith Black pigment contains pure neutral carbon black together with a toluene soluble carrier resin such as Stabilite Ester 10 of the Hercules Powder Co., U.S.A.

MICROLITH BLUE 4GT comprises a stable phthalocyanine blue pigment with a greenish cast together with Stabilite Ester lO resin.

MICROLITI-I GREEN GT comprises a medium shade of phthalocyanine green together with Stabilite Ester 10 resin, the microlith pigments are manufactured by Ciba Co., Switzerland.

Colour Index of the Pigments Microlith Blue Colour Index No. 74160 Microlith Green Colour Index No. 72455 ELVACITE RESIN is an acrylic resin manufactured by DuPont, Delaware, U.S.A.

Coates hydrocarbon dispersible flake black comprises pure carbon black together with ethyl hydroxy cellulose resin.

B.P.V. OIL synthetic automotive lubricating oil containing antioxidant ZDP, dialkyl zinc dithiophosphate in solution, made by British Petroleum Ltd.

KOHINOOR CARBON BLACK supplied by A.C. Hattrick Ltd. Aust.

SUNFLOWER SEED OIL vegetable oil supplied by Meggitts Ltd., Australia.

HOSTAPERM BLUE B3G copper phthalocyanine blue, pure beta-form, made by Hoechst, C.I. pigment Blue 15, Colour Index No. 74160.

GRAPI-ITOL BLUE BLF, phthalocyanine blue, C.I. pigment blue made by Sandoz.

' PERMANENT YELLOW 66, extra, a diazo yellow pigment without lake forming groups. C]. pigment yellow 17, colour Index No. 21105.

BRILLFAST ROSE RED 4444, a red phosphotungsto molybolic acid toner.

ISOL RUBY RED BKS 7520 (KVK) a lithol ruby red CI. Pigment Red 57, Agfa, Calcium lake PALE LOWERING LITI-IOGRAPHIC VARNISH a polymerized linseed oil varnish made by Meggitts Ltd., Australia, Polylin /3, acid value 40-65 viscosity 7.0-9.5 poises at 25C, from alkali refined linseed oil.

RHODENE RESIN L42/70, a safflower oil modified alkyd resin made by Polymer Corporation, Australia, acid value 6-10, with 69-71, percent solids, 64 percent oil length.

ISOPAR G a hydrocarbon liquid solvent with greater than percent isoparaffinic content, and aromatics and olifins less than 1 percent, and remainder cyclo and normal paraffins, KB No. 27, final boiling point 177C.

ISOPAR E a hydrocarbon liquid solvent with greater than 95 percent isoparaffinic content, aromatics and olifins less than 1 percent with remainder cyclo and normal paraffins, KB Value 29, final boiling point 143C.

B880 100 is an aromatic hydrocarbon solvent with 98 percent aromatics, KB value 9', final boiling point 182C.

We claim:

1. The method of developing electrostatic images wherein the electrostatic image is formed on a photoconductive surface which has light struck relatively conductive image areas and dark-adapted charge bearing areas, and wherein the liquid developer contains a carrier liquid having suspended in it a relatively insulating primary developer compound which is more insulating than a secondary more conductive developer compound also suspended in said carrier liquid, said primary compound having a polarity in said insulating carrier liquid which causes the primary developer to migrate to said charge bearing areas to develop the same by electrical attraction, the more conductive developer compound being particulate and closer in polarity to the charge bearing areas so that it can be urged away from said photo-conductor surfaces when a bias is applied through the said developer but is driven toward it when the polarity of the bias is reversed, said method comprising the steps of a. subjecting said surface to said liquid developer having the two said dissimilar developing compounds, while maintaining a bias field through the said liquid of a polarity such that, as the image is developed by the first developer compound due to said electrical attraction, the second developer compound is moved away from the surface being developed, and

b. reversing the polarity of the biasing field when the charge bearing area has been developed differentially on the said surface by the said primary compound to cause the second developer compound to be deposited on the first developer compound, whereby as the primary developer compound has a slower charge exchange rate and is retained on said image, the more conductive developer compound is retained only on the relatively insulating surface formed by the deposited first developer compound which inhibits ready charge exchange.

2. The method of claim 1 wherein the photoconductor is zinc oxide which is negatively charged and the first developer compound is a copolymeric resin having a relatively positive charge in an insulating carrier liquid, and the second developer compound is a particulate toner with an added medium to render the same conductive.

3. The method of claim 1 wherein the photoconductor is one which can be negatively charged, and the first developer compound is a copolymeric resin having a relatively positive charge in an insulating carrier liquid, and the second developer is carbon black surrounded with a fixing resin and calcium octoate to render the same relatively conductive.

4. The method of claim 1 wherein the first biasing is of a lower voltage than the second biasing but is maintained for a longer time.

5. The method of claim 2 wherein the first biasing is about 20 volts applied for about thirty seconds and the second biasing is about 180 volts applied for about two seconds.

6. The method of claim 3 wherein the first biasing is about 20 volts applied for about 30 seconds and the second biasing is about 180 volts applied for about 2 seconds, 

2. The method of claim 1 wherein the photoconductor is zinc oxide which is negatively charged and the first developer compound is a copolymeric resin having a relatively positive charge in an insulating carrier liquid, and the second developer compound is a particulate toner with an added medium to render the same conductive.
 3. The method of claim 1 wherein the photoconductor is one which can be negatively charged, and the first developer compound is a copolymeric resin having a relatively positive charge in an insulating carrier liquid, and the second developer is carbon black surrounded with a fixing resin and calcium octoate to render the same relatively conductive.
 4. The method of claim 1 wherein the first biasing is of a lower voltage than the second biasing but is maintained for a longer time.
 5. The method of claim 2 wherein the first biasing is about 20 volts applied for about thirty seconds and the second biasing is about 180 volts applied for about two seconds.
 6. The method of claim 3 wherein the first biasing is about 20 volts applied for about 30 seconds and the second biasing is about 180 volts applied for about 2 seconds. 